According to the Terms and Conditions of Marriott's Look No Further(SM) Best Rate Guarantee, the guarantee does not apply to rates available on auction websites. Our research considers this type of re-sale site "auction" in nature. You are purchasing from an individual through a third party. The rate is determined by the individual and not consistently offered.

https://www.cancelon.com/faq
How does it work?

Seller posts a hotel reservation on Cancelon.
2. Buyer selects a reservation Buyer wants to buy.
3. Buyer buys the reservation via Cancelon. Seller will get the payment a day after the check-out day, to prevent Seller from selling the hotel reservation to more than one Buyer.
4. Both Seller and Buyer should follow rules and formalities of the hotel.

For this reason, we must deny your claim under the terms of Marriott's Look No Further(SM) Best Rate Guarantee program.